1. Field of the Invention
The present invention relates to a compression mold tool for use in the compression molding of plastic articles. More particularly, the present invention relates to a mold tool for use in the compression molding of threaded plastic articles, such as plastic closures, which allows the article to be mechanically unscrewed from the mold core.
2. Description of the Related Art
Several methods exist for forming plastic articles. Some of the more common and efficient methods for forming high volumes of plastic articles involve molding, either by injection molding or compression molding, where the plastic article is formed within a mold of the article.
Where injection molding involves the injection of high temperature, molten plastic into a mold, compression molding takes place by compressing a warm plastic pellet between mold parts to form the article. Some patents which describe the compression molding process and apparatus for carrying out compression molding include U.S. Pat. Nos. 5,554,327; 5,603,873; 5,770,130; 5,866,177; 5,989,007; 6,074,583 and 6,123,880.
As disclosed by the aforementioned United States patents, apparatus for compression molding articles generally comprise a plurality of molding tools mounted in a circumferential array on a rotatable turret in a plurality of opposed mold parts in tooling pairs. The upper and lower tooling of each pair carry opposed male and female mold parts that together, when closed, form a cavity mold for compression molding the desired article.
The turret is mounted on a machine frame that also carries cams for operative engagement with the tool parts. The cams move the mold parts of each tooling pair toward each other during a portion of the rotation of the turret around the turret axis, for compression molding an article between the tooling pairs, and move the tooling pairs away from each other during another portion of the rotation of the turret. The separation of the tool parts exposes the article molded between the upper and the lower mold parts for release of the article from the upper mold part.
After the mold parts separate, an actuator pushes down on a stripper sleeve to physically push the cap off of the upper mold part. Because the stripper sleeve merely pushes the threaded article off of the mold part, without unscrewing the threaded article from the threads on the upper mold part, the parameters of the threaded part are limited.
The type of compression molding apparatus currently in use generally allow the user to mold closures with a thread profile that is not less than about 30xc2x0 to 35xc2x0 and a thread projection of less than about 0.045 inches. Closures with lesser thread profiles or greater thread projections generally cannot be compression molded because they will be damaged by the cavity stripping cycle involved in the operation of conventional apparatus.
Thus, it is desirable to provide a mold tool for molding threaded plastic closures with lesser thread profiles and greater thread projections than those heretofore possible. It is also desirable to provide an improved mold tool that can be retrofitted to and used with existing compression molding apparatus. It is also desirable to provide a mold tool that eliminates the need for specialized rotary compression molding equipment. It is the object of the present invention to meet said needs.
These and other objects are achieved by the present invention which is directed to an improvement in a compression mold tool for forming a threaded plastic article having an upper mold part on a stationary core, a lower mold part and a stripper sleeve which is actuated for pushing the article off of the upper mold part, said improvement comprising a rotating inner sleeve which engages the article and a threaded portion on the stationary core for turning said rotating inner sleeve when said stripper sleeve is actuated.